Fixed speed running means for automatically maintaining a running speed of a vehicle at a constant speed have been provided conventionally. During running of a vehicle at a constant speed by such constant speed running means, it is frequently desired to change the running speed to a different constant speed or to accelerate or decelerate the vehicle.
When the running speed of a vehicle is to be changed to another constant running speed or the vehicle is to be accelerated or decelerated during running at a constant running speed, the aimed running speed of the vehicle for constant speed running is raised or an accelerator pedal of the vehicle is treadled.
It is to be noted that, when a vehicle is running, for example, following another preceding vehicle on a road where traffic is heavy, the running speed must be changed particularly frequently (acceleration from a constant speed running condition). Therefore, here in Japan where traffic is heavy, a system for maintaining the running speed of a vehicle at a constant speed is required to have a good performance in changing the running speed from a constant speed running condition.
By the way, when the running speed of a vehicle in a constant speed running condition is to be changed or when a vehicle is to be changed from a constant speed running condition to an accelerated or decelerated running condition, it is necessary to set the acceleration or deceleration of the vehicle to a suitable value. In particular, when a driver accomplishes a manual operation to change the running speed of the vehicle, smooth changing of the running speed can be attained by operating an accelerator pedal or the like while normally perceiving the running condition of the vehicle. However, where a vehicle is to be accelerated or decelerated by automatic control of an engine, such acceleration or deceleration may not be achieved smoothly depending upon setting of acceleration or deceleration and may cause a shock or hunting, which deteriorates driving feeling of the driver.
The present invention has been made in view of such a subject as described above, and it is an object of the present invention to provide an engine controlling system for a vehicle which can attain smooth changing of the running speed of the vehicle when the running speed for a constant speed running condition is to be changed or when the vehicle is to be changed over from a constant speed running condition to an accelerated or decelerated running condition.